In a proposal made to the 3GPP2 standards body, (Framework proposal for LBC mode of Rev C, C30-20060625-054R1, a contribution of a joint proposal for loosely backward compatible FDD mode of HRPD Rev C made by China Unicom, Huawei Technologies, KDDI, Lucent Technologies, Motorola, Nortel Networks, QUALCOMM Incorporated, RITT, Samsung Electronics, and ZTE, dated Jun. 26, 2006), a system is described that integrates pre-coded Code Division Multiple Access (pre-coded CDMA) and Orthogonal Frequency Division Multiple Access (OFDMA) on the uplink between a Mobile Station (MS) and a Base Station (BS). This hybrid CDMA-OFDMA uplink is intended to enable more efficient support of different types of traffic. Typically, it is proposed that this hybrid system will enable best-effort traffic at relatively high data rates to be transported on the OFDMA air-interface while delay-constrained traffic will be carried over the CDMA air-interface.
As is well known, CDMA is a scheme whereby multiple users are distinguished by their use of unique spreading codes. The actual spread sequence can be carried either by amplitude or phase modulating a pulse waveform (Direct-Spread [DS] CDMA or pulse-shaped CDMA), or on a set of orthogonal sinusoidal tones (Multi-Carrier [MC] CDMA). In the latter case, the spread sequence can also be coded prior to modulating the orthogonal tones. This type of multiple-access scheme is the afore-noted pre-coded CDMA and, when the coder is the Discrete Fourier Transform (DFT) matrix, it can be called DFT-CDMA.
OFDMA is a scheme whereby a unique set of sinusoidal frequencies is assigned to each user within a sector. MC- and pre-coded CDMA differs from OFDMA in three respects. Firstly, more than one user may transmit over the same set of frequencies in the MC- or pre-coded CDMA system. Secondly, different pilot structures may be used in these two cases to enable coherent detection. Thirdly, by an appropriate choice of the pre-coder in the case of pre-coded CDMA, the Peak-to-Average Power Ratio (PAPR) may be controlled to be lower than for an OFDMA transmission.
Since multiple users may transmit on the same frequencies, a CDMA link does not require scheduling by the base station to allocate and de-allocate frequencies to a particular user. Thus, CDMA permits autonomous operatio on the uplink and is particularly suited for initial random access, transmission of bursty information, and traffic with tight delay constraints. In the case of an OFDMA system, frequency resources must be allocated and de-allocated (or allocated and maintained), which entails both additional delay as well as use of forward link (FL) power and bandwidth resources to communicate these grants. A system in which frequency resources need be allocated and maintained can disadvantageously result in an inefficient use of reverse link resources since frequencies may lie unused for substantial periods of time.
In accordance with the present state of the art, in a hybrid system, such as the proposed 3GPP2 system, a new packet of user data is transmitted on the uplink by a mobile station in accordance with the type of traffic the data represents. Thus, as noted above, best-effort traffic at relatively high data rate will be transported on the OFDMA air interface, while lower-rate traffic, such as Voice Over Internet Protocol (VoIP), will be transported on the CDMA air interface where it can be transmitted autonomously and avoids the delay and bandwidth expenses of frequency allocation required by OFDMA. FIG. 1 is a high-level block diagram of a conventional hybrid mobile station 101 in accord with the afore-noted 3GPP2 standards proposal, which integrates both a pre-coded CDMA encoder and modulator 102 and an OFDMA encoder and modulator 103. Switches 106 at the input and outputs of encoder/modulators 102 and 103 are commonly switched so as to encode and modulate a new user data packet 104 via pre-coded CDMA or OFDMA in accordance with the type of traffic the packet represents. The CDMA- or OFDMA-encoded and modulated user data packet is then amplified and transmitted on the uplink 107 by amplifier and transmitter 108.
As noted, pre-coded CDMA and OFDMA differ in their PAPR characteristics. A pre-coded CDMA transmission has a lower PAPR than an OFDMA transmission. This difference in PAPR translates directly into a different maximum average transmission power for the two air-interfaces. For example, for the same power amplifier, where a mobile station may be able to sustain transmission at an average power of 200 milliwatts (mw) while transmitting using pre-coded CDMA, it may be able to sustain an average transmission power of no greater than 120 mw while using OFDMA. This is a result of the fact that the PAPR for an OFDMA transmission is higher, and in order to maintain the power excursions above the average that are similar for both OFDMA and pre-coded CDMA, the maximum average transmit power needs to be lower for OFDMA. The practical implication of this lower allowable average transmit power for OFDMA transmissions is that the cell radius for such OFDMA transmissions is smaller than for CDMA transmissions, or equivalently, a lower data rate is supported at the edge of sectors that were laid out for existing DS-CDMA systems. The conventional solution to alleviate this problem requires that either expensive PAPR reduction techniques be applied to the OFDMA waveform, or more powerful and therefore more expensive power amplifiers be used.
A less expensive methodology is thus needed in order to match data rates at a cell edge between existing DS-CDMA systems and the new proposed hybrid OFDMA-pre-coded CDMA systems.